excessive valve lash needed

Hello,

I have a 72 B that requires at least .017 lash to idle smooth and bring vacuum up from 10 to about 14. Then I can tune the carbs to idle nice and the engine seems to burn cleaner. I really don't know the history of the engine but that it is an 18GB. The head was rebuilt 2 years ago. I have even compression at about 140.

I have a new dizy and have tried many different timming positions.

I re-checked timming chain sprocket alignment and valve position per shop manual to make sure that the cam is not advanced or retarded.

Here is something that concerns me. I did find the following when checking my cam lobes in the engine.
Dial indicator showed a .262 lift. Which tells me that it is just a bit worn. But when I tested for "inlet valve opens at 16*" rule, I find that it starts to open sooner. At about 25* or so.
I think the cam has a longer duration because of this test? Therefore would this be the reason for a better idle at .017 lash?

Sorry but I know just enough about engines to be dangerous!

Thanks for your suggestions,
Rob.

I THINK, and I mean I Think,
That if you indeed have a cam with a greater duration, it causes a rougher idle. Therefore vaccum is lower. Hence more lash simulates (somewhat) advancing the cam.

Hopefully the experianced guys can help here.

OK, if that is the case and I do not want to get in there AGAIN and advance the cam, is .017 lash detrimental to the valve train? (and not only to my ears:)

Rob

Put the lash back at 13 for now. Then measure the lift of the cam. If you have a higher lift cam, it should have a lumpy idle. Too much lash will cause damage to lifters and the camshaft. Rob, Are you in Northern California? You can bring it over and I'll be glad to help do some fun, weekend diagnosis. Basil

You don't mention if the cam is aftermarket or stock, I've seen aftermarket cam with recommended lashes of .022"-.024". Elgin cam for example always have more recommended lash than some other grinders

hap, i don't believe he knows what cam is in it.

Start at the beginning and make sure your TDC mark is accurate. Then what is your timing set at? Retarded timing will act like you describe.

Thanks guys, I'm in So Cal.

That is correct, I don't know what cam it is, but like I mentioned, I checked the total lift with a dial indicator and read .262. I did this with the indicator sensing the top of the push rod. However, it does appear that the duration is greater. But this test was only as good as my testing to see at what point BTDC it started to open.

I have checked several times to make certain that #1TDC is correct with timing pointer, and it is.

I have tried timing from 5-25* BTDC. Anything from 15-25 gives me more vacuum, but still not over 12 pounds. But I will not run it that high because she will run lumpy as soon as I bring the RPM up. I get will over the 3500 RPM to 35* rule, via vacuum and centrifugal advance.

When driving, the car does seem to run fine. But I guess that may be difficult to really feel.

Question, If there is a concern about running to much lash, what happens with cam owners that require .022-.024" ?

"Question, If there is a concern about running to much lash, what happens with cam owners that require .022-.024" ? "


Good question....